uppressed (Schleifer et al., 1984; Schleifer, Keller, Bond, Cohen, & Stein, 1989). Further, depressed mood is associated with immune system suppression among people with AIDS who are not undergoing bereavement (Kemeny et al., 1994). The studies examining immune system suppression in people undergoing various types of environmental stress only sometimes identified the subjective psychological states that are probably the mediating state connecting environmental events to immune system function. In the studies on human beings, brain regions activated in psychological states that then influence the immune system have not been assessed. However, given the findings from the animal literature, it is reasonable to speculate that activation of the periaqueductal gray mediates the impact of environmental stress on immune suppression. PSYCHOLOGICAL HEALTH AND IMMUNE SYSTEM ENHANCEMENT A natural opioid substance, beta-endorphin, has been identified in the literature on immune system enhancement (Anton), Schneiderman, et al., 1990). Although there are findings to the contrary (Sacerdote, Manfredi, Bianchi, & Panerai, 1994), and although the relationship may be curvilinear rather than linear (Fiatarone et al., 1988), at particular levels betaendorphin has been associated with increased natural killer cell activity (Levy et al., 1991; Mandler, Biddison, Mandler, & Serrate, 1986). A number of behavioral and psychological interventions are known to alter endogenous opioid levels. Systematic desensitization, the treatment for helping individuals overcome phobias, increases beta-endorphin levels (Thyer & Matthews, 1986). Opioid antagonist drugs block the efficacy of systematic desensitization, suggesting that successful desensitization is mediated through endogenous endorphins (Arntz, Merckelbach, & de Jong, 1993; Egan, Carr, Hunt, & Adamson, 1988; Merluzzi, Taylor, Boltwood, & Gotestam, 1991). The self-efficacy of people who overcome phobias appears to ...
